CA1036572A - Process for preparing materials having improved mechanical and thermal properties and materials obtained thereby - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

Process for preparing materials constituted by oxides, having improved mechanical and thermal properties characterized in that materials constituted by oxide are treated by a silicon compound of general formula:

Description

:
3~;572 The present invention relates to a process for obtaining materials having a high thermal and mechanical stability, constituted by metal oxldes substantially free from silicon oxides and, more particularly, alumina which is used as catalys-t or as catalyst carrier for chemical reactions in heterogeneous phase.
It is known that, when chemical processes are based on chemical reactions carried out by means of catalysts in heterogeneous phase, the catalyst undergoes irreversible transformations which decrease the efficiency thereof and a frequent replacement o the catalyst itselE is needed. The process become more convenient when the rate of such transformations is decreased, which increases the cat~lyst efficiency employment period.
Generally the heterogeneous phase ca-talyst ac-tivity increases on increasing the catalyst surface in contact with the reagents. On this purpose catalysts having a high porisity and surface area or catalysts carried on materials having such characteristics are used.

~owever the increase of the porosity and the surface area of the carriers and/or catalysts is generally obtained to the detriment of their physical characteristics and, as a consewuence, such catalysts or carriers undergo a quick deterioration when employed, which causes a progressive reduction oE the catalytic efficiency.
Particularly this problem is remarkably observed in the case of reactions carriecl out in presence o~ catalyst in so called fluid beds and chemical reactions occuring in the catalytic muffler of cars wherein the catalyst undergoes also a more or less quick decay due to the abrasion raising from the impact of the granules among one another and against ., ,~ .

~ 36572 the ~all~, consequent of the vibrations due to the car working~
When these materials are brought to high tempe-rature~, the aforesaid phenomena are further overburdened:
the increase of the mechanical thrusts ~oined to the ther-mal ones causes, ~rom one hand~ a higher abrasion and mecha-nical breaks and~ from the other hand9 a decrease of the : surface area because of the sintering. Particularly such a problem occurs when these materials are used as catalyst . 10 carriers in the car mufflers for the removal of the noxious components of the waste gasesO In fact the temperatures achievable in the car catalytic mu~flers are very variable and~ sometimes on working~ -they may ac~lieve ~lue~ hi~her than 1000~; in ~hese cases the u~ual materlals used a~
carriers undergo thermal tran.~ormations vJhich remar}cably decrease the physical properties thereof and become very sensitive to the mechanical thrusts; moreover a shrinkage of the volume thereof can be observed due to a sinteriza-tion which affects the effective volume of the container, .-that ~urtherly overburdens resistance problems to compres~
. sion and abrasion. :~.
Hitherto methods have been proposed ~or improving .
the surface characteri~tics, ~or instance o~ a umina bv ~:
means of a treatment with alkaline~ alkaline-earth metal compounds or with silicic acid or colloidal silica, but the obtained results ha~e been .not satisfactor~.
~ he present invention purposes to provide a method for improving the mechanical properties o~ alumina .
; ox.ides substan~ially free from silicon oxides through a simple and economical treatment~
.According to a further purpose of the present invention, modified alumina having a high thermal and '~.

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mechanical stability are provided.
~ nother purpose of the present invention consists of the production of alumina useful as catalyst or as catalyst carrier for reactions in heterogeneous phase.
According to a particular purpose of the present invention there is provided a method for the production of alumina granules having high mechanical and thermal properties useful as ca-talyst carriers to be used in the ca-talytic mufflers of cars for -the removal of noxious components in the waste gases.
The above ones and other purposes are achieved by the present invention through a simple and economical process consisting in impregnating the al~lltlina subs-tantially Eree ~rom siLicon oxides, the mechAnical characteris-tics of which are to be improved, with a silicon compound, subjecting the so impregnated material to a thermal -treatment and subjecting the resulting product.
The inventive process may be carried out in liquid phase, under atmosphere or overatmosphere pressure.
The process carried out in liquid phase and under atmospheric pressure consits of an impregnation, at room tempe-rature, of the solid alumina with a silicon compound as such or in solution with other organic compounds having a boiling point preferably lower than the one of the silicon compound and which are chemically compatible therewith (hydrocarbons, alcohols, etc.), and a heating o~ the so obtained composition at temperature ranging from 50 to 500C. Particularly a diluent is ilsed when stated amounts, lower or less, of the silicon compound are to be deposited on thc material which is to be modified. The proccss carried out in vapour phase and under atmospheric pressure consists in introducing a chosen silicon compound in a gas stream and _ _ _ _____ _ __ _ __ _ _ _ , !

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feeding such a mixture onto the ~olid material at temperatures ranglng from the silicon compound boiling temperature and 500C, preferably 100 and ~00C~
A pre~erred embodiment of the process o~ the in-vention, when high amounts of the sil.icon compounds are to be deposited~ is one wh~ch is carried out in liquid-va-pour phase and under overatmo~phexic pre~sure in an autocla- ;:
ve at presRures comprised from the atmospheric pressure to 50 K ~ cm2, and preferably from 10 to ~0 ].g/cm2 and at tempe-ratures ranging from the room temperature to 500C~ prefe-rably from 100 to 400C over a time of from 1 to 20 h.
The al umina treated a¢cording to the three afore~ald proce~ses~ iq furtherly sub~ected to a thermal treatment under an inort ga~ stream at a temperature ranging from the boillne temperature of the silicon compound to 500C over a period ranging from 1 to 5 h; the alumina iB at last treated with air at a temperature ranging from 300 to 600C over a period ranging from 2 to 10 h.
~he thermal treatment under an inert gas atmosphere allows the remo~al~ throu~h carbonization or pyrolisis, of the most part of the organic ~aterial~ which remains bound to the carrier9 and9 therefore, the follo~ing treatment with air does not promote local overheatings, which would bring about noxious modifications to the obtained product.
Preferably the employed air is mixed with an inert gas~
; as nitro~en~ in order to control the oxidation temperatuxe.
Alternatively the treatment with nitrogen may be replaced by a slow air oxidation~ very diluted by an inert ~asO
~he obtained alumina has high mechanical and thermal characteristics and a production cost not higher than .
the one of the compounds which are not -treated with ~.

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the silicon compound.
The silicon compound which are employable according to the invention has the general formula X
Y - Si - W

in which X, Y, Z and W may be ~-R), (-OR), (-Cl~, (-Br), (-F), (-SiH3), (-COOR), (-SiHnClm), / osi (OR)2_ 7p osi (OR)3; (-R) being ~, an alkyl, cycloal~yl, aromatic, alkyl-aromatic or alkyl-cycloalkyl radical having from 1 to 30 carbon atoms, such, for instance, CII3, -C2H5, isopropyl, n-propyl, n-butyl, isobutyl, cyclohexyl, cyclopentyl, phenyl, phenilcy-clohexyl, alkylphenyl; n, m and p being ~hole numbers comprised from 1 to 3.
~ mong the above cited compounds, orthosilicic acid esters are preferred, such as, for instance, methyl, propyl, iso-propyl, isobutyl and normal butyl tetrasilicates.
~` Other metal oxide could be treated according to the inventive process, such as titanium oxides, magnesium oxides, chromium oxides, zirconium oxides, iron oxides or mixtures of these oxides with one another or with other compounds.
The products obtained by the inventive process have very higher mechanical and thermal characteristics than the ones of the starting products and, show, at chemical and physical-chemical anaylyses, surface structures and ; compositions substantially different from the startin~ ones.
In fact the infrared spectrum undoubtedly chan~es and the ; bands, charac-teristic of the silicon - hydroxyl band, pre-vail. Particularly in the case of alumina treated by the inventive process~ bands at 3795 and 3737 cm 1 oE the difEerent : hydroxyl groups present on the alumina surface - -,'~ , .: , _ 5 _ ;`' ~1~

are no more observed, bu~a band at 3745 cm 1 characteristic of the hydroxyl ~roup bound to silica is observed~
In the ~ollowing description o~ operative examples ;~
a re~erence will be made~ for the purpose o~ explaining but not limiting the invention~i of the application o~ the inven~
ti~e process to materials constituted by aluminium oxides~
~ore particularly a re~erence vrill be made. to proces~e~ for the stabilization of ~-alumina ~lhich is used ~ .
as carrier ~or catalysts of mar~y chemical reactions carried ;.: :
out on indu~trial scale~ and al~o to the so obtaLned stabi- :
lized aluminas which sho~r9 a~ter a heating at 1200C over a period o~ 24 hour~ only a change ~rom ~ phas~ to ~ phase.i :
Sa~d alum~nas~ stabili~ed by a ~reatment over 40 hour~ at ~..
250C at a steam pre~sure o~ 15 ~tm, keep u~ohanged their ¢r~talline ~tructure, their exceptional mechanical and thermal characteristics and their super~icial area~
~oreo~er ~alumina~ ~tabiliæed according to the invention a~ter trea~ment at 1000C o~er a périod o~ 24 hours~
~ho~s a volume shrinkage lower ~han 2 ~XAMoe~
~ ~p~eroidal ~Al203 ga~ ~as prepared according :.
to the process de~cribsd Ln USP 3~416~8~ in the na~e o~ the . same applicant; ~ ~ :
It consi~ted in dipping a mi~ture o~ ammonium ace-~, .
~at~ and alumL~ium chlorohydroxides ~ith a suitable gelling . .
agent into a mineral oil kept at gooa~ ~ ~
On a column bottom~gel little spheres were reco~e-red which~ suitably treated ~vith ~3 and ~as~ed with H20, ` .; ~.
cr~stallized i.nto ~ nonohydrate~ ~he iittl~ spheres, dr.ied 30 a~d then calcined, were changed Lnto ~ -~l203~ ~he resis-tance again~t abrasion be~ore and a~ter ther~al treatment "
at 7000 and 1100C ~or 24 hours~ the ~olume shrl~ re a~d `~3 - 6 -3~572 th~ surface area variations after analogous treatment o~
alumlna a~ a~ore~aid obtained Yere examinedO ~he determina- :
tio~ o~ the resistance agaln~t the abrasion was carriea out by employ~ng a steel con~ainer~ having t8 cc ~olume, ~hich wa~ ~illed~ ~or 80~ with the material to be exam~ed.;
A hig~ frequency vibratio~ over 30~ wa~ given ~o said co~tainer by mean~ of a 5uitable apparatus.
Before the beginn~ng of the test~ the sample to b~ examined wa~ kept in stove at 150~ ~or 2 hour~ was co.oled ~t xoom temperature under an anhydrous atmosphere and then was weighed very care~ullyO~ At the end of the abrasion te9ts~ the recovered sample was ~ieved in order ~o remove the gra~ule ~raotions~ blo~ed by air ~or removing the;p~der adheri~g the lit~le ~phere~ a~a~ dried at 150~ ~or 20 h a~d then ~elghedO
~he ~riotion resis~ance (K) ~va~ expressed as wei~ht percentage los3 o~ the sample.' - :
~ he results obtained ~ro~ the various determina-t~o~ are reported in table lt .. . . . .
~ 20 E~PIE 2 .~
Accord~ng to the procedure of example one~ a spheroi_ dal ~ A1203 was prepared containing 3~ SiO2 ~b).i ~he product v~s obtained by adding colloidal silica ~udux ~trade mark) AS (Du Pont) to mixture o~ ammonium-acetate~
. aluminium chloro-hydroxide and a ~elling agent.~
. On the ~ample so obtained determ~nation~ o~ ~, ~ur~ace area~ volume ~hrin~age and abr~sion after a thermal ; . treatment at 1000C and 1100C were carried out; the results o~ these determinati~ms are reported in ~abl~
3Q E~AM:P~E 3 :4 . A sample ~ -Al20~ that used in e~ample 1 t~ras added b~ Ba in the îollolr~ing Yray:

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tO0 g o~ alumi~a ~ere imp~egnated ~ith a solution obtained by dls~olving g. 9.8 o~ ~a (X0~)2 into 80 cc. of H~0 A~ter dry m g at 120C ~or 12 hour~ and an air ~al~ination at 500C ~or 2 hour~, a ~ -A1203 was obtai~ed cbntaining 5~2% of ~a (c)-. ~he determinations carried out .
o~ th~ ~o obtained sam~le are reported in ~able 1.

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- ~y employing again.the same alumLna tha~ used in - :
example 1~ 100 g of A1203 were immersed i~ 200 cc of (C2X50~4~i and kept co~acting the li~uid ~or 4 hours; at end .the ~olid was separated ~rom the li~uid in ~xce~
an~ was tran~erred lnto a quartz pipe put in an electrie ~ve~; a nitrogen stream Y~S sent and the vhole waa ~10rJ1Y
heated up to the boiling temperature o~ ethylorthosllica~e (160.-170C) ~o that to completely distillate the unreacted product, ~he thermal treatment ~as ~hen prosecuted up to ~ '`
500C~ when the ni~rogen ~low wa3 stopped and air ~as sent;
the duration of the:~~inal treatment wa~ 2 hours. A product ~as obtained having a SiO2 content equal to 6.1% (d)~ ~he -~ -:ZO results o~ the thermal treatments and the other determlnation~ !'`. ` -per~ormed on the so obtaIned sample are reported in table 1 In table 1 4he results o~ X-ray examination . carried out on the samples of example~ 1~ 3 and 4 at 1100C
and t200C are al~o reported. :-.
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~3~,5r72 By comparing the data of Table 1 it is po~sible to infer . :~
that the treatment o~ a ~ -Al203 with Si (OC2H5)4 cause~
a stabilizing effect very higher than the one of the other known common methods~ .
~XAMPIE 5 A spheroidal ~ -alumina sample was prepared accor-ding a rotating plate pelletizing technique in the ~ollowing way: ~ -Al2037 reduced into a very fine powderD was put in a rotating plate; while the plate was rotating, an aqueous solution containing 0.1~ of hydrated methyl cellulose (Methocal) was nebuli~ed onto the powder itself; sph~eroidal nuclei ~ormed, the 8iZQ~ 0~ which could be regulated accor_ ding to the staying period in the plate and th~ alumina pow-der present therein. When w~h~d size~ have been achie~cd~
the rcsulting alumina spheroids were dried ~or 24 hour~
at 120C~ then air cal¢ined up to 500C (e). ~he characta-ri~tic~ o~ these aluminas are illu~trated in table 2. A :.
sample o~ this alumina was immersed in an excess of (C2H50)4 ;
Si; according to a same process that ln example 3~ a ~-A1203 was obtainad containing 6.3~ of SiO2 (f). This sample too - ~
wa~ sub~ected to slnterization test~ and the obtained result~ ^ .
.; are reported in table 2~ . :
EXAMP~E 6 100 g o~ ~ame alumLna o~ example 5 were put in a self-heating autoclave togethex with 4Q g of (C2H50)4 Si, ~he autoolave was e~acuated and more and more ~a~hed with N2 in order to remove all 2 trace~; then it was charged under a 5 l~jcm2 pressure with N2. '~he autoclave was heated at 200C and thi~ temperature was kept ~or 4 houxs; at end it ~0 was cooled~ the pre~sure was released and alumina was reco~
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vered~ whioh was sub~ected to a followi~g thermal treatment :.
o~ 2 hour~ at 200C under N2, and then to an air calcination ~
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~L~36~72 at 500C ~or 4 hoursO ~i -The ~ -A1203 little spheres, in such a way trea-ted, after analysisJ ~howed a SiO2 content equal to 10~2~o (g) The results of the sinterization tests are listed in table 2.

Same alumina o~ example 5 was treated with CH3Si (OC2H5)3 in the vapour phase at room pressure in the following way: 100 g of alumina were charged in a quartz pipe immersed in a heating oven; the pipe was bottom connec-ted to a two necked flask~ containing 30cc o~ methyltrietho-xylane~ and immersed in a thermostatic bath. Alumina was heated at 400a under a nltrogen stream; thl~ temperature having been aohleved ln the alumina bed~ the thermos-tatio bath was brought up to 120C and N2 was ~ent to the flask containing CH3Si (OC2H5)3 till the total vaporization of the silicon compound.
~he treatment at 400C was prosecuted with air for 4 hours, then the whole was cooled.
The SiO2 conten~ of the so treated alumina was equal to 8.5~ (h). ~he sinterization tests gave the result3 listed in table 2.
EXAMP~E 8 100 g of same alumi~a o~ example 5 were impregna-ted with an aqueous solution of orthosilicic acid obtained ~n the ~ollowing way:
25 cc o~ sodium silicate ~40 Bé) were kept in 70 oc f H20; the solution was contaoted with an ionic exchange resln (Amberlite IRC -50 ~I~) in order to completely remove of Na~ ions.
After the cationio exchange~ the solution was ; ~ trademark 365~7Z

utilized for impregnating aluminaO After drying at 120C
and air calcination at 500C for 4 hoursS an alumina was obtained having a SiO2 content equal to 605~ (i).
The results o~ the slnterization tests, which the sample was sub~ected to, are listed in table 2.
qP~ 9 Another sample of same alumina of example 5 was im-pregnated with colloidal silica ~udox S~q (Du Pont) in the ~ollowing ~vay: `
7 g of eolloidal silica at 30~ were diluted in 80cc of H20; 100g of alumina were impregnated with the resulting solution.l After drying at 120a .~or 12 hours and calcination at 500a in air ~or 4 hours~ an alumina vias obtained containing 1.6~o of SiO2 ( 1 ) The result~ o~ the tests carried out on this samples are listed in table 2.
~XAMPLE 10 A sample of same alumina that used in example 5 vJas treated with SiC14 in vapour phase according to th~
following way: 100g of A1203 were put into a quartz pipe immersed in an electric oven; a nitrogen stream was sent and -the sample was treated till to 400C; now ~he pipe was conne~ted to a saturating vessel containing SiCl4 kept . . .
at room temperature, through whioh an anhydrous nitrogen stream flowed, which was then sent onto the alumina sample.
kfter four hours of treatment~ the nitrogen ~low through the saturatin¢ vessel oontaining Sial4 wa~ ~topped~
and air was sent, After one hour air treatment the vessel was cooled and alumina was reoovered which~ at analysi~ ~howed a SiO2 content equal to 7.3'~ (m) ~he so obtained ~ample, ~ubjected to sinterization tests~ gave the results reported in table 2.
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~365~72 A drawing was per~ormed o~ ~ame alumina prepared according to the proces~ dlsclo~ed in example 5~ which wa~
treated with (CH30)2SiC12 in the ~Gllowing way: 100g o~
Al203 were charged into a ~uartz pipe immersed in an elec-tric oven; the pipe was connected to a N2 stream and was treated till to 200C; a~ter 2 hour~ the pipe was connected to a satura~ion ~e~sel ¢ontain.ing (~H30)2SiC12~ kept at 60C~ .
and through which an anhydrou~ N2 stream wa~ passed.
After 4 hours of this treatment~ the vapour stream :~
was ~topped and air wa3 sent; the temperature was raised to 500~ and the air treatment wa~ pro~ecuted ~or 4 hour~; :
at end the ~e~el was cooled and the material (n) wa~ reco-vered~ whlch wa~ sub~eoted to varlou~ te~t~ in order to evaluate the the~mal ~tability and meohan~cal characberi~-tlcs thereo~; the re~u~ts o~ the performed te~ts are repor~
ted in tabl~ 2.' ..
EXAMP~E 12 . , By u~ng ~ -Al20~ of example 1~ two tablet~ were prepared ~uitable to be examined at I.R. : :
~he first tablet was treated with ethyl orthosilicate in the ~ame conditio~ and according to the procedure of example 6, the second one was treated with colloidal silica ~udox S.M. (~u Pont) according to the procedure of example .
9. ~he 50 prepared t~o sample~, after dehydration under .. -vacuum at 450C~ were examined at I.R. and the ~pectrum . `;
is reported in the ~igure~ 1 and 2~ ~herein the abscis~ae .
refer to the ~requency o~ the in~rared radiatlon expres~ed in cm 1 and the ordinates refer to the percentage transmis-sion.
In the first case (fig. 1) was obtained an absorp-tion ~pectrum typical o~ silica wherein a ver~ clear band trademark 3 i _ -~ ~36~72 was observed at ~745 cm 1-f a~.~rded to Si-OH group with a disap~eararLce of b~nds at 3737 c~ 1 and 3795 cm 1 and a s~rong attenuation of ba~d~ at 3698 cm l awarded to Al-OH band.
In the second case (fig.2) wa3 obtained an over- ;;
lapp~ng adsorption ~pectru~ t~pical o~ a mixture of si1ica ..~
and alumina, this latter being the prevailing one. .
EXA~PIES 13 - 18 100 g of the same alumina of example 5 were put in a self-heating autoclave~ The autoclave ~Jas evacuated and more and more washed by nitrogen i~ order to remo~e all oxyge~ trac~s: then,:time by tlme, the ~o~lo~ing amount~
of ~ilicon compounds were introduced:
: Example ~3 . g 30 o~ dieth~lchlorosilane (~2Hs)2sia~2 14- g 17 o~ tetram~thylsilane (C~ )4S~
15 g 17- o~ acetoxisilane : X3Si (OOCCH3) 16 ^~ g 18: of methoxidisilane CH30SiH2 (SiH3) 17 g 22 of triethylsilane (C2H5)3siH
18 g 45 o~ polymeth~lsiloxane (C~ )3SiO( ~ )2 - SiOSi(C~ )3 ~he pressure was brought to 5 Kg/cm b~ nitrogen. .
~he autoclave ~ras heated up to 200G over 8 hours;
a~ end it ~a~ cooled, the pre~sure ~as released and alumina `
~as recovered~ which was heated Ln a quartz pi~e under a . nitrogen stream for ~ hours at 200C~ and then air calcined at 500C Por 4 hoursO
; . ~he characteristics o~ these modi~ied aluminas are emphasized in table ~, ~rhere are collected the results of :,: .
the measurement~ carried out after the thermal ageing treatment at 1100C for 24 hours.
~or purpose of comparison~ the dat~ obtai~ed ~ith a _ sample prepared accordLn~to example 1 (a) w~re reported alsoO , '", "
E$A~2~E 19 100 g of same alumina used Ln example 1 were immer-sed into 200 cc o~ (C2H50)4 Si and kept contacting the liqui~
for one hour; at end the solid ~las separa~ed from ~he liquid i~ exce~s a~d ~as tr~n~er~ed into a quartz pipe im~ersed i~ an electric oven; a nitrogen stream ~s sent and the 't ~ ' whole ~as heated up to the boiling tenperature of ethylor-thosilicate~ so that to completely distillate the unreactedproduct O
A~ter the end o~ the ethylorthosilicate distil- ..
lation~ the nitrogen ~10VJ was ~topped~ air v~as sent and the heating ~a~ pro~ecuted by gra~ually ~ncrea~ng the temp~ra-. ture; ~Yhen this reached 350C, determined on alumina~ a . combu3tion reaction ~as started o~ the organic groups bound to the alumina surface and on the condensation products . i.
: . thereo~ there~or the temperature fastly raised to 900 -1000 C . .. ;;
20. The violent combustion occurring negatively affect~
the physical and mechanical characteristics o~ thP fInal ; :
product, a~ it is possible to Infer from the data of table ~ ~
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EXAi2L~ 20 By using two samples of ~ -alumina~ ~he fo~mer ~ :
: prepared according to example 1 and the latter accordin~
to example 4~ hydrothermal treatments were carried out, at : -growing times, at 250C9 300C and 350Co 15 g of the two samples were introduced into two tests tubes~ hich v~here ~ .
. . .
introduced into 0O5 l autoclave whereto 10 cc of water ~ere addedO
The autoclave ~emperature ~s brought to the ; 10 stated temperature (250C, 300~, 350C) ana the pressure wa~ re~ulated at 15 atm steam by means o~ the manometer ~uitably connected to the au~ocla~e~ possibl~ by releasin~
the pressure exces~ throu~h the ~u~table valve.
~ ~he sur~ac~ ar~a of the samples~ in ~uoh a r~
.~ treated o~er t~mes ~rom 4 bo 64 hourq i3 reported in the diagram o~ ~ig.l3~ wherein the abscissae refer to the treatme~t duration and the ordinates relate to the ~ur~ace . area (m~g). It i3 possible to note that the stabilized alumina (curve_1) does not undergo any modificationg whereas 20 alumina as such (gy3~5 ~ ) undergoes progressive ~-decreases of the sur~ace area which meansg as confirmed by X-ray ~nalyses, a change, more and more higher, o -alumina into aluminium monohydrates~; . ;
,, ~ he curve 1 refers to the three temperature va~ue~ whereas the curve 2 refers to 350C~ the curve 3 to 300C and the curvc ~ to 250C.i .
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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Process for the preparation of alumina having improved mechanical and thermal properties, characterized in that the alumina is subjected to the following treatment:
- impregnation in liquid or vapor phase with a silicon compound of the general formula in which X, Y, Z and W may be (-R), (-OR), (Cl), (Br), (-F), (-SiH3), (-COOR), (-SiHnClm), -[OSi(OR)2]p OSi(OR)3, (-R) being H, and alkyl, cycloalkyl, aromatic, alkyl-aromatic or alkyl-cycloalkyl radical having from 1 to 30 carbon atoms, n, m and p being whole numbers comprised from 1 to 3, - thermal treatment of the impregnated alumina under an inert gas stream at a temperature ranging from the boiling temperature of the silicon compound to 500°C over a period of from 1 to 5 h; and - controlled oxidation of the resulting product with air at a temperature ranging from 300°C to 600°C over a period of from 2 to 10 h.

2. Process according to claim 1 characterized in that (-R) is selected from the group consisting of -CH3, -C2H5, iso-propyl n-propyl, n-butyl, isobutyl, cyclohexyl, cyclopentyl, phenyl, phenylcyclohexyl, alkylphenyl radicals.

3. Process according to claim 1 characterized in that the silicon compound is an alkyl orthosilicate.

4. Process according to claim 1 characterized in that the impregnation with the silicon compound is carried out in the liquid phase at atmospheric pressure and at temperature comprised between 50 and 500°C.

5. Process according to claim 1 characterized in that the impregnation with the silicon compound is carried out in the vapour phase at atmospheric pressure by flowing the silicon compound in an inert gas stream onto the material which is to be treated.

6. Process according to claim 1 characterized in that the alumina which is to be treated and the silicon compound are contacted at pressures ranging from the room one to 50 kg/cm and at temperatures ranging from the room temperature and 500°C, preferably from 100 to 400°C.

7. Process according to claim 1 characterized in that the controlled oxidation is carried out under an air stream at 500°C.